The principal aim is elucidation of the immunogenetic and phylogenetic basis of allograft incompatibilities. This will include several distinct, but conceptually related projects as follows: (1) Immunologic phylogeny and phylogeny of transplantation reactivity in particular. We shall continue to study primitive fishes (especially hagfish) and selected, advanced invertebrates (especially echinoderms and protochordates). (2) Weaker histocompatibility barriers in congenic strains of mice to identify geneantigen-antibody relationships as a function of known histocompatibility genes. (3) Attempts to further characterize exceptions to the genetic rules of transplantation in mice as a function of known gene combinations. (4) Occurrence and mechanisms of immunological enhancement in both normal and neoplastic tissue test systems employing genetically defined animals.